The human heart is divided into four chambers. These include the right atrium, the right ventricle, the left atrium, and the left ventricle. The right atrium and right ventricle are divided from the left atrium and left ventricle by a muscular wall called the septum. The atrial septum is the wall separating the atria, and the ventricular septum is the wall separating the ventricles.
Early in fetal development the two atria (i.e., left and right atriums) are a single chamber. A wall or membranous structure develops from the superior aspect of the atrial chamber and extends superiorly toward the base of the atrial chamber. This membrane is the septum primum (SP). As the SP seals to the base of the chamber, it is dissolved away at the superior attachment, creating a passageway for blood to travel from the right atria to the left atria (bypassing the developing lungs). At about the same time, a second membrane develops from the superior aspect of the right atrium and extends inferiorly. This membrane is the septum secundum (SS). It fuses with the SP along the walls of the atria, but does not extend to the base of the atria. The inferior portion of the SS is named the limbus. The two membranes form a passage defined by thin tissue (SP) and thick tissue (SS) that extends from the right atria to the left atria. This passage is named the foramen ovale. The portion of the SP that comprises the left side of the foramen ovale is named the fossa ovalis. The limbus of the SS is distinct from the fossa ovalis of the SP in that it is thicker and more muscular.
Upon birth blood must be diverted into the lungs of the newborn. One event that enables this is an increase in pressure within the left atrium relative to the right atrium. This pressure reversal effectively closes the foramen ovale and eliminates the shunting of blood from right to left. In most people, the SP and SS membranes that form the passage of the shunt fuse and the passage is eliminated. However, in a minority of people, these membranes do not fuse effectively and the shunt remains sealed by pressure, but the passage remains viable, or patent. This condition is named patent foramen ovale (PFO). In unusual circumstances the pressure in the right atrium can exceed that in the left atrium, allowing passage of blood through the PFO. This would typically be inconsequential, except when the venous (right atrial) blood contains thrombotic debris that is normally eliminated by thrombolytic mechanisms in the lungs. In this case, a clot can travel to the left atria and become an embolic risk to the patient's health through myocardial infarction or stroke.